The leukocyte group-5 antigenic system has 2 dominant alleles( 5a and 5b) which segregate independently of HLA. We have made a discovery that the 5a allele of this system has a striking association with acute lymphoblastic leukemia(ALL). This is the strongest association yet found of an antigen with human leukemia. We propose to: 1) study the association of 5a with ALL in an attempt to gain insight into the cause of ALL; 2) determine the significance of the 5a antigen as a prognostic marker; 3) study patients with other hematologic disorders for increased frequency of the 5a&b alleles, and 4) attempt to assign the group 5 locus to a particular linkage group.